1. Field of the Invention
The present invention relates generally to a process for forming a glass coating on a silicon substrate and products made thereby, and more specifically, to a process for forming a relatively thick layer (e.g., 25-250 microns) of electronics grade glass on a silicon substrate and to products formed thereby. Particular utility for the present invention is found in the area of fabrication of electronic components, such as printed circuit boards, although other utilities are contemplated, such as fabrication of semiconductor and microwave devices and multichip modules.
2. Brief Description of Related Prior Art
Printed circuit boards (PCBs) are used extensively in the electronics arts to mount and interconnect discrete electronic components (integrated circuit chips, etc.) to implement a specific function. Commonly, the board substrate is made of silicon, although other materials, such as polymers and ceramics are also used. Often, it is desirable to electrically isolate (insulate) certain components mounted on the PCB from each other and from the board substrate. Typically, this is accomplished by applying one or more layers of dielectric material to the surface of the board and then mounting the components on or in the dielectric layers.
Many processes exist in the prior art to form such dielectric layers on a silicon substrate. Common techniques for forming dielectric layers include chemical vapor deposition (CVD) and spin-on-glass (SOG) techniques. These processes are essentially limited to producing dielectric layers of only several microns in thickness, due to their inability to effectively prevent cracking of layers of greater thicknesses and/or their prohibitively slow formation rates. This is unfortunate, since in many applications it is critical that capacitive coupling between the substrate and the components be made as low as possible, and dielectric layers of only several microns in thickness are inadequate to provide this degree of capacitive isolation. Additionally, many of these processes are incapable of forming a dielectric layer having a sufficiently high dielectric constant (e.g., at least 4.1 at 20 degrees C. and 1 MHz) and sufficiently low loss tangent (e.g., at most 0.06 percent at 20 degrees C. and 1 MHz) to permit use of the resulting board in high performance electronics applications (e.g., microwave and/or other radio frequency circuit applications).
Prior art processes also exist for producing a relatively thick layer of dielectric such as glass on a silicon substrate. However, in general, in these processes, the thermal expansion characteristics (i.e., thermal coefficient of expansion) of the glass are not adequately matched to those of the substrate. Since circuit boards must be able to operate over a wide temperature range, any mismatch between the thermal expansion characteristics of the substrate and the glass can cause mechanical stresses that can result in deformation of the substrate and mounted components, cracking of the glass, etc.
U.S. Pat. No. 4,133,690 to Muller discloses one example of a process for coating a silicon substrate with a dielectric glass composition. The glass composition is applied to the substrate in the form of a finely ground powder and thereafter fused onto at least a portion of the substrate. The glass may comprise borosilicate. In one example, the glass powder is applied to the substrate in the form of an aqueous suspension of about 250 microns in thickness. The suspension is then dried to remove the carrier fluid and fired to melt the glass. The resultant glass layer is about 200 microns thick.
Disadvantageously, during the Muller process (and similar prior art processes, including prior art "tape glass" techniques), the edges of the glass film can pull away from the substrate, due to the inherently high surface tension characteristics of melting glass. That is, during melting of the glass particles, their inherently powerful tendency to flow together can become greater than their relatively weaker tendency to bond or adhere to the surface of the substrate. This can cause the resulting glass layer to be non-uniform (i.e., the glass layer may be discontinuous, and may contain unintended void pockets, and/or other deformities). This condition can seriously degrade the electrical properties of the glass layer, and the resulting PCB.
Other problems also plague prior art processes. Some processes apply a layer of metal and/or other conductive material to the substrate prior to deposition of the glass material and firing of the PCB. Disadvantageously, it has been found that during firing of the PCB, the conductive layer often reacts with the underlying silicon to form a composite material. This can also result in serious degradation of the electrical properties of the conductive layer and the resulting PCB.
Similar prior art processes are disclosed in Goldstein et al, U.S. Pat. No. 4,093,771 and Prabhu et al., U.S. Pat. No. 4,369,220. These processes suffer from the aforesaid and/or other disadvantages.